1. Field of the Invention
The present disclosure relates to a method of manufacturing a rotor core (pole core) having a plurality of claws of an electric rotating machine.
2. Description of Related Art
It is known to use, as a rotor of an electric rotating machine such as a vehicle-use alternator, a pair of pole cores each having a cylindrical boss section formed with an axial hole thereinside, a disk section radially extending from one end of the boss section, and a plurality of claw sections axially extending from the periphery of the disk section at equal intervals toward the other end of the disk section. Each adjacent two of the claw sections form a V-shaped channel reaching the outer periphery of the disk section.
The pair of the pole cores are disposed opposite to each other such that the claw sections of one pole core are located in the V-shaped channels of the other pole core with a certain clearance therebetween. A field coil is disposed between the outer periphery of the disk sections and the inner periphery of the claw sections, and a shaft is pressure-inserted into the axial hole to constitute a Lundell-type rotor core.
Such a pole core is manufactured by cold forging or hot forging in view of productivity, as shown, for example, in Japanese Patent No. 3609745. This patent document describes a method of manufacturing a pole core as shown in FIG. 7, in which a boss section 2a is placed downward, an axial hole 2b is punched through the boss section 2a using a punch p of an upper mold m3a and a center die d of a lower mold m3b, and simultaneously with this, claw sections 4a are ironed to stretch while being bent until they form an angle of 90 degrees with a disk section 3a so that step portions 4a1 of the claw sections 4a are pressed against step portions f2 of the lower mold m3b, and shaped into step portions 4b. 
In this method, the axial hole 2b is formed in one pressing step using the punch p. Accordingly, since an amount of slide movement is large, the shaping accuracy may be lowered depending on the accuracy of slide movement. Further, since a pressing load is large, a large apparatus is necessary to manufacture the pole cores, and accordingly the productivity is low. Further, since the boring punch p is a part of the upper mold while the punch for shaping the boss section is a part of the lower mold, displacement between the centers of the upper and lower molds is likely to increase, causing displacement between the center of the boss section and the center of the axial hole to increase.
Japanese Patent No. 2935817 describes a stepped-hole forming method as shown in FIG. 8, in which an item to be shaped is formed with a through hole H1 with an inverse-tapered portion using a shaping punch having a smaller diameter portion, a tapered portion and a larger diameter portion, and then formed with a stepped hole H2 having a tapered shape by pressing work.
According to this stepped-hole forming method, to form the stepped hole H2 having a larger diameter using the shaping punch p1, the through hole H1 having a smaller diameter is formed at first. Accordingly, in this method, excess metal Y easily flows into the smaller diameter portion and the hole edge by a large amount during pressing work. Since such excess metal Y interferes with a shaft inserted into the axial hole or a bearing collar at the time of assembling the pole cores, the resistance load of pressure-insertion of the shaft increases, and the fitting strength of a bearing is lowered.